Delta Opioid Receptor Agonist Protocol

UFP-512 was synthetized by Balboni et al. (2002) (link), SFN was acquired from Merck Chemicals and Life Science S.A.U (Madrid, Spain), and naltrindole and naloxone were purchased in Sigma–Aldrich (St. Louis, MO, United States). UFP-512 was dissolved in saline solution (0.9%) and intraperitoneally administered at 1, 3, 10, 20, and 30 mg/kg. SFN was dissolved in dimethylsulfoxide (1% in saline solution 0.9%) and intraperitoneally administered at 10 mg/kg. naltrindole and naloxone were also diluted in saline solution (0.9%) and subcutaneously administered at 4 and 1 mg/kg, respectively. All drugs were prepared daily just before use and administered in a final volume of 10 ml/kg, at 1 h (UFP-512), 3 h (SFN), and 30 min (naltrindole and naloxone) before testing in accordance to our previous pilot studies and other works (Aguila et al., 2007 (link); Hervera et al., 2013 (link); Carcolé et al., 2014 (link); Redondo et al., 2017 (link); Ferreira-Chamorro et al., 2018 (link)). For each group treated with a drug, the respective control group received the same volume of corresponding vehicle.